MEMS (microelectromechanical system) are flexible cantilever-type structures that have many applications and an even greater potential in today's advancing technologies. MEMS are formed using semiconductor processing technology and are formed over substrates that may be formed of semiconductive or insulating materials. For example, MEMS may be deflectable mirror structures that can reflect light in different directions. In recent years, the projection-display industry has undergone a period of explosive growth. Until several years ago, such projection display systems were predominantly based on either cathode ray tubes (CRT) or active-matrix liquid crystal display (LCD) technology. All of these traditional display systems, however, suffer from limitations that compromise their performance or the spectrum of their applicability. LCD- and CRT-based systems are limited in their ability to support high-brightness applications, and they suffer from uniformity and stability problems in large-scale applications.
An emerging projection display technology called Digital Light Processing (DLP) accepts digital video and transmits to the eye a burst of digital light pulses that the eye interprets as a color analog image. Digital Light Processing is based on a MEMS device known as the digital micro mirror device (DMD) invented in 1987 at Texas Instruments Inc. The DMD is a fast, reflective digital light switch that combines with image processing, memory, and a light source and optics to form a digital light processing system. The DMD is a light switch that uses a plurality of electrostatically controlled MEMS mirror structures to digitally modulate light, producing high-quality imagery on-screen.
The MEMS used to form the plurality of light switches are typically formed over CMOS memory devices and using CMOS-like process. Each light switch includes a deflectable aluminum alloy mirror that can reflect light in different directions depending on the state of the underlying memory cell. The deflectable mirror of the MEMS, commonly an aluminum alloy, is formed over a releasable layer also referred to as a sustain layer. The sustain layer is formed between the mirror film and the substrate and is removed after the mirror layer has been formed into one or a plurality of discreet mirror structures that are anchored to the substrate to form cantilever-type MEMS. Several patterning and etching operations are used to pattern the mirror and other films formed over the sustain layer and the etching operations are typically plasma etching operations that etch down to the sustain layer which functions as an etch-stop layer. A preferred, easily removable sustain layer material is amorphous silicon, commonly represented as α-Si. The α-Si used as the sustain layer, however, is prone to attack during the wet photoresist strip/clean processes carried out at high temperatures such as are used in conjunction with the etching operations referred to above. If the α-Si sustain layer is attacked, the structural integrity of the MEMS may be compromised.
It would therefore be desirable to provide a method and structure for producing a MEMS mirror structure using an α-Si sustain layer that is not susceptible to attack.